US banks such as Goldman Sachs Group, Morgan Stanley and Bank of America are asking investors to disclose whether they plan to borrow extra to invest in significant risk transfer (SRT) vehicles, as regulators examine whether they pose a threat to financial stability, according to people familiar with the matter. Investors have been asked this question in recent months as the promotion process kicks off. It comes as authorities warn investment firms looking to buy SRT, a transaction that helps banks sell the risks associated with their loan portfolios, to leverage. Because the debt is supplied by another bank, credit risk is in part simply being transferred to another part of the financial system.